FastAPI always returns content, even if 204 no content status code is set.

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Example

from fastapi import FastAPI
from starlette.status import HTTP_204_NO_CONTENT
app = FastAPI()


@app.get("/", status_code=HTTP_204_NO_CONTENT)
def read_root():
    print("d")

Description

Send a request with any HTTP Client, observe the traffic using tcpdump/wireshark - you’ll see that you get null as a response (content-length will be 4). This is extremely notable as this caused this issue https://github.com/encode/httpx/issues/1474 (The client doesn’t expect any data in that case) I would expect FastAPI to not return null when response is None.

Environment

• OS: macOS
• FastAPI Version [e.g. 0.3.0]: 0.63.0
• Python version: 0.63.0

[falkben]

You could instead set the response class for that path operator to be Response instead of the default JSONResponse. Then it won’t convert None into JSON. So w/ your example above it would be:

@app.get("/", status_code=HTTP_204_NO_CONTENT, response_class=Response)
def read_root():
    print("d")

which correctly returns b"".

note, if you are doing this on multiple endpoints, you can set the default response class on your router, or even your entire app. https://fastapi.tiangolo.com/advanced/custom-response/#default-response-class

[aviramha]

Thanks @falkben. I found this solution also but I believe it should be handled by the framework. It’s fairly simple to implement hence the PR I submitted. Returning 204 with data is invalid according to the standards so FastAPI should detect such scenario and handle it accordingly IMO